The present invention relates to methods for the labeling of nucleic acid polymers in vitro and in vivo. Certain methods are provided that include a [3+2] cycloaddition between a nucleotide analogue incorporated into a nucleic acid polymer and a reagent attached to a label. Other methods are provided that include a Staudinger ligation between a nucleotide analogue incorporated into a nucleic acid polymer and a reagent comprising a substituted triarylphosphine attached to a label. Such methods do not require fixation and denaturation and therefore can be applied to the labeling of nucleic acid polymers in living cells and in organisms. Also provided are methods for measuring cellular proliferation. In these methods, the amount of label incorporated into the DNA is measured as an indication of cellular proliferation. The methods of the invention can be used in a wide variety of applications including clinical diagnosis of diseases and disorders in which cellular proliferation is involved, toxicity assays, and as a tool for the study of chromosomes' ultrastructures.